The disclosed subject matter relates generally to manufacturing and, more particularly, to a bootstrap supply for a switched mode power converter.
Switched mode power converters are widely used to provide voltage, current, or power controlled power sources for various loads. For example, a power converter may control output voltage thereby acting as a voltage regulator that converts an input voltage to a desired output voltage. A power converter may also provide a constant current source to power a load, such as a light emitting diode (LED) array.
Typical off-line switched mode power converters (i.e., powered from AC mains) include a high voltage supply that is routed through a load circuit by means of a switch or switches. The load circuit includes one or more dissipative elements, and may include inductive elements that store energy for later delivery to the dissipative elements within the load circuit. A typical load circuit may include one or more resistors, diodes, light emitting diodes (LEDs), capacitors, inductors, transformers, terminal connections, switches, etc., and/or various active circuits, etc. A switching transistor is coupled between the load circuit and a high voltage supply terminal for controlling the amount of energy supplied thereto. When the load circuit is coupled across the high voltage supply, power is supplied to the load circuit. When the load circuit is uncoupled from across the high voltage supply by the switching transistor, the load circuit operates in isolation from the high voltage supply. If the load circuit contains an inductive element, then the inductive element may supply power to the dissipative elements while the load circuit is isolated from the high voltage supply. A load feedback parameter (e.g., voltage, current, or power) is often monitored to determine the load requirements. The duty cycle of the switching transistor is controlled to meet the load requirements, the duty cycle representing the fraction of the main power conversion switching cycle in which power is supplied to the load circuit.
The control circuitry of a high voltage power converter often requires a supply voltage that is lower in magnitude than the high voltage supply. Simple regulators have been employed that derive the low voltage supply from the high voltage AC mains using a dissipative series element, such as in the circuit of FIG. 6A. A controller 600 controls a first transistor 605 for powering a load 610 and a second transistor 615 for charging a capacitor 620 that provides the control voltage, VCC. The arrangement of FIG. 6A wastes an appreciable amount of power, reducing the efficiency of the power converter. For example, for a power converter controller operating from a peak-detecting bridge on 220V European mains, the high voltage supply can approach 360V. The power converter may draw over 10 mA while driving the gate of the switching transistor at high switching frequencies, so the power wasted can exceed 3.6 W (i.e., 10 mA×360V).
To avoid this efficiency loss, other techniques have been employed to generate the control voltage supply by using a more efficient, auxiliary switch mode power supply. For example, as shown in FIG. 6B, a winding can be added to a magnetic element 625 in the main switched mode power converter circuitry thereby creating a parasitic transformer secondary from which a low voltage supply can be generated using the capacitor 620 and a diode 630. However, this approach complicates the construction of the magnetic element, and often precludes the use of off-the-shelf magnetic components, resulting in increased cost. Furthermore, since the auxiliary supply is crudely derived from the main power converter, the precision of the low voltage supply is decreased. When the low voltage supply is higher than necessary, power is wasted.
Another technique employed to generate the control voltage supply involves the use of a high-voltage switching element 635 connected between the switched terminal of the load 610 and the capacitor 620, as shown in FIG. 6C. This technique avoids significant efficiency loss, but it increases cost by requiring an additional high-voltage transistor and associated control circuitry. High-voltage transistors are far more costly than low-voltage transistors. Furthermore, high-voltage transistors are far more difficult to integrate within a monolithic integrated circuit than low voltage transistors.
This section of this document is intended to introduce various aspects of art that may be related to various aspects of the disclosed subject matter described and/or claimed below. This section provides background information to facilitate a better understanding of the various aspects of the disclosed subject matter. It should be understood that the statements in this section of this document are to be read in this light, and not as admissions of prior art. The disclosed subject matter is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above.